1. Field of Invention
The invention pertains to a camera flash control circuit and particularly to a flash control circuit with reduced luminance error of a generated flash by use of a phototransistor.
2. Related Art
With the progress digital image processing technology and charge coupling devices (CCDs), digital cameras have become very important. Since computers have become widely used and other kinds of digital technology are improving rapidly, the digital camera has found itself many more applications. With these advantageous contributions, our daily life is pushed rapidly to become more digitized.
The flash is an indispensable part of a camera. Current flash light operation has a preflash stage and a main flash stage. The former is performed for determining the luminance of an object to be photographed. Next, the determined luminance is used to determine a required luminance of the flash in the main flash stage. Specifically, the time for the preflash is considerably short, e.g. 3 μs. Then, a CCD and a central processing unit (CPU) in the camera are used to compute the required luminance of the flash and the corresponding time for the main flash stage, which is typically 0.1 ms to 3 ms. As such, the flash may be controlled to match the required amount of the exposure in any environment.
For example, U.S. Pat. Nos. 6,441,856, and 6,359,651 and Taiwan patent No. 570206 adopt the two-stage flash technology. However, since the flash is generated by discharge of a large capacitor charged with a high voltage, such as 160 uF/330V, the preflash stage may reduce the maximum energy which may be released at the main flash stage.
Taiwan patent 562986 also disclosed an apparatus and a method for controlling a flash, in which no preflash stage is needed. In the apparatus, a light-adjustment circuit is used to perform an opto-electrical transformation for the reflected light from an object to be photographed, i.e., incident light emitted from the flash, to output an exposure voltage. When the flash voltage is greater than the reference voltage, the flash ceases to emit a light, which means that the luminance of the flash is sufficient. On the other hand, when the exposure voltage is lower than the reference voltage, the flash keeps emitting light.
Although the technology in this patent may improve the disadvantages in the prior art, it still has some technical problems that need to be improved. For example, the flash requires an additional discharge signal, and lacks the function of forcing flash generation. In addition, the control circuit is composed of bi-polar junction transistors, which leads to a slower response speed, a larger transmission delay and has an inclination to over-exposure and imprecise flash control.
Furthermore, too much error may occur in current technology. In a luminance test of a single camera after multiple preflashes, the luminance falls between 130 and 160 and thus has an error of about 10%. Therefore, the technology using preflash to determine the required exposure is not precise enough.
A precise flash control may not be necessary for previous digital cameras with fewer pixels. However, this is not the case for currently used cameras with more pixels, in which the flash control has a considerable effect on the resulting image quality. For precise flash control, many factors must be considered such as number of components, operating range of the voltage and signal delay. Therefore, developing a more precise flash control technology using fewer components has become an urgent need in the technology field.